In the field of microelectronics, and in particular the fabrication of microelectronics during plasma etching processes, electrostatic chucks have been used to hold silicon wafers during the plasma etching process. Current electrostatic chucks that operate by the "Johnson-Rahbek Effect" consist of a metallic base plate that is typically coated with a thick layer of slightly conductive dielectric material. A silicon wafer of approximately the same size as the chuck is placed on top of the chuck and a potential difference of several hundred volts is applied between the silicon and the base plate of the electrostatic chuck. This causes an electrostatic attraction proportional to the square of the electric field in the gap between the silicon wafer and the chuck face. When the chuck is used in a plasma filled chamber the electric potential of the wafer tends to be fixed by the effective potential of the plasma. The purpose of the dielectric layer on the chuck is to prevent the silicon wafer from coming into direct electrical contact with the metallic part of the chuck and shorting out the potential difference. On the other hand, a small amount of conductivity appears to be desirable in the dielectric coating so that much of its free surface between points of contact with the silicon wafer is maintained near the potential of the metallic base plate; otherwise, a much larger potential difference would be needed to produce a sufficiently large electric field in the vacuum gap between the wafer and chuck. Typically, the face of the chuck has a pattern of grooves in which about 10 torr pressure of helium gas is maintained. This gas provides cooling (thermal contact) between the wafer and the chuck. A pressure of 10 torr is equivalent to about 0.2 psi.
Commercially available electrostatic chucks have many shortcomings. The holding pressure within prior art "Johnson-Rahbek" chucks is quite sensitive to the conductive properties of the dielectric layer--meaning they can be quite temperature sensitive. The dechucking (or release) time is slow in these prior art chuck designs due to the time required to discharge the free surface of the dielectric coating through the high resistance of the coatings, thus extending the processing time for each wafer. The dielectric coatings on these chucks typically are made of a carefully formulated mixture of ingredients and are easily damaged by abrasive forces generated during wafer clamping and by accidental plasma exposure. Furthermore, wear or degradation of the dielectric coating can introduce undesirable foreign material into the plasma chamber. Lastly, these prior art chucks require a large electrical potential, typically several hundred volts, in order to cause the electrostatic attraction necessary to hold the wafer.
The prior art also includes chucks which are made from monolithic slabs of single-crystal silicon. These chucks are custom processed into an array of flat topped protrusions which are elevated 50 micrometers or more above the remainder of the etched silicon surface. The silicon wafer being held is supported by these protrusions. These prior art chucks were not intended to be used in wafer processing steps which require robust wafer cooling; the gap between the wafer and the chuck face in the area surrounding the protrusions is too large for efficient thermal conduction by means of low-pressure helium gas. This large gap was intended, instead, to facilitate the use of vacuum to generate some or all of the holding force.
The most serious shortcomings of these prior art chucks arise from the fact that essentially all of the electrostatic holding force would be generated within the area occupied by the protrusion tops which are in contact (or very nearly so) with the wafer being held. The high fields from which the attractive force arises would be present only in that portion of the chuck face due to a relatively thin, conformal coating of silicon dioxide dielectric which is applied to the entire surface of the chuck face after the array of protrusions has been created. Charge that is unintentionally transferred between the wafer and the chuck face protrusions, either by direct electrical contact or by tunneling and field emission in regions of near contact, would result in locally reduced fields and an associated reduction in the attractive force. De, chucking could also be adversely affected; upon removal of the applied potential difference to allow dechucking some of the transferred charge may remain on the surfaces of the protrusions and cause the wafer not to be released. Lastly, if helium gas cooling were attempted with these prior art chucks, higher electric fields would be required to generate sufficient holding force, compared for example to "Johnson-Rahbek" chucks, because the force arises from a small fraction (typically 25% or less) of the total area of the chuck face--which would greatly increase the likelihood of problems from field emitted charges because field emission current follows a rapidly increasing function of electric field strength.
Thus, there is an existing need for a silicon electrostatic chuck for microelectronic fabrication that will overcome the shortcomings of the prior art by providing an electrostatic chuck wherein a conductive dielectric coating or expensive silicon processing, and high voltage, which give rise to most of the problems of the prior art, is not needed.